I love ArM5, but sometimes I feel like whoever wrote the rules should have been given lessons on technical writing. They are so imprecise.
What are we to make from the he "Vis Use Limit" (p. 94)?
"The amount of raw vis that a magus may use in a single season is limited to twice his Magic Theory score. The magus cannot successfully integrate any more vis into a single project."
I think it's clear that the amount of raw vis that a magus may use in a single season is not meant to be limited at all. He can cast multiple rituals without running into this rule; this rule is in the the Laboratory chapter and is meant to relate only to laboratory activities. Hence, I suggest a reading of
The amount of raw vis that a magus may use in a Laboratory Activity is limited to twice his Magic Theory score.
The Basic Laboratory Activities section is particularly irritating. It talks about "laboratory tasks" instead of Laboratory Activities, and says "If an activity is based on a Lab Total, the magus needs a laboratory to do it. If it is not, he does not" which is contradicted by the next section on "Creating a Laboratory" and the nearby "Fixing Arcane Connections". Regardless, I suggest that
A Laboratory Activity is any activity described in the Laboratory chapter that requires a season of laboratory work
in accordance with the first paragraph, "... seasons, each of which is enough time to accomplish a single long-term [L]aboratory [A]ctivity".
Now we can finally turn to longevity rituals.
"The ritual takes a season, and culminates in some sort of focus... The Longevity Ritual takes one season to develop and perform... "
so it seems that the Vis Use Limit applies here: to develop and perform the Longevity Ritual you need to spend a season in laboratory work.
Yet also
"You can ... perform the ritual from the old ritual again. This involves simply making a new investment of vis (of an amount based on your current age) but no significant investment in time. You must have the Laboratory Text... from the original ritual to do this, and this is the only benefit from a Longevity Ritual's Laboratory Text."
Clearly, "the ritual takes a season" is inconsistent with the rest and should be read as developing and performing the ritual takes a season, with the development being a seasonal Laboratory Activity and performing the ritual taking but little time.
Even under the unmodified rules, performing it from the original ritual does not require a Season so need not fall under the Vis Use Limit anymore than casting a ritual spell does (although the latter falls under the "Limit on Vis Use in Spellcasting" on page 82).
It can be argued that a Laboratory Activity should be counted as any activity involving a Lab Total, as the text on "Basic Laboratory Activities" implies, opaquely. This I see as a too narrow definition, but even if accepted it isn't clear that performing the ritual from the old ritual again requires a Lab Total. Nothing seems to suggest it - the old Laboratory Text for example doesn't add to your Lab Total as it normally does. The rules are completely silent on the matter. A Lab Total is related to seasonal activities, this is a momentary activity.
I don't agree that performing the Longevity Ritual falls under the "Limit on Vis Use in Spellcasting", however. That applies to spellcasting, there is no indication that performing the Longevity Ritual is spellcasting.
It seems to me that the rules meant to say something like this: developing a Longevity Ritual is a Seasonal Activity that is much like inventing a new spell, requiring no raw vis. The Longevity Ritual must then be performed, which takes (say) a day and requires the raw vis. The amount of raw vis is unbound, there is no intention to limit it much like there is no intention in limiting the amount of raw vis in Vis Transfer (page 94) IMO. The cost in raw vis is supposed to be prohibitive as of itself.
This is by no means a solid conclusion, of course. I think that's what the authors meant to write, but I ain't certain. The rules at any rate are inconclusive, vague, imprecise, and self contradicting. House Rules/interpretations are necessary.
As for what is desirable, which is perhaps more important, I like having Magic Theory matter as per "The Vis Use Limit Applies" camp, but don't like the implication that great potions are gotten at young age as they cannot be made for old magi. All in all, I think the other interpretation (that the limit doesn't apply) cheapens Longevity Potions a bit, but works better.
